If you guess something, you give an answer or provide an opinion which may not be true because you do not have definite knowledge about the matter concerned.

If you guess that something is the case, you correctly form the opinion that it is the case, although you do not have definite knowledge about it.

A guess is an attempt to give an answer or provide an opinion which may not be true because you do not have definite knowledge about the matter concerned.

If you say that something is anyone's guess or anybody's guess, you mean that no-one can be certain about what is really true.

You say at a guess to indicate that what you are saying is only an estimate or what you believe to be true, rather than being a definite fact.

You say I guess to show that you are slightly uncertain or reluctant about what you are saying.

If someone keeps you guessing, they do not tell you what you want to know.

You say guess what to draw attention to something exciting, surprising, or interesting that you are about to say.